What is active, reactive and apparent power - a simple explanation

Definition

The load on the circuit determines how much current flows through it. If the current is constant, then in most cases the equivalent of the load can be determined by a resistor of a certain resistance. Then the power is calculated according to one of the formulas:

P = U * I

P = i²* R

P = u²/ R

The same formula determines the total power in the AC circuit.

The load is divided into two main types:

• Active is a resistive load, like - TENOV, incandescent lamps and the like.
• Reactive - it can be inductive (motors, starter coils, solenoids) and capacitive (capacitor units, etc.).

The latter happens only with alternating current, for example, in a sinusoidal current circuit, which is exactly what you have in sockets. What is the difference between active and reactive energy in a simple language so that the information becomes clear to beginner electricians.

Sense of reactive load

In an electric circuit with a reactive load, the current phase and voltage phase do not coincide in time. Depending on the nature of the connected equipment, the voltage either outstrips the current (in inductance) or lags behind it (in capacitance). To describe the questions using vector diagrams. Here, the same direction of the voltage and current vector indicates the coincidence of the phases. And if the vectors are depicted at a certain angle, then this is the lead or phase lag of the corresponding vector (voltage or current). Let's look at each of them.

In inductance, voltage is always ahead of current. The "distance" between phases is measured in degrees, which is clearly illustrated in vector diagrams. The angle between the vectors is indicated by the Greek letter Phi.

In an idealized inductance, the phase angle is 90 degrees. But in reality, this is determined by the full load in the circuit, but in reality it can not do without a resistive (active) component and a parasitic (in this case) capacitive one.

In capacitance, the situation is the opposite - the current is ahead of the voltage, because charging inductance consumes a large current, which decreases as the charge. Although more often they say that the voltage lags behind the current.

Briefly and clearly, these shifts can be explained by the laws of switching, according to which the voltage cannot change instantly in the capacitance, and the current in the inductance.

Power Triangle and Cosine Phi

If you take the whole circuit, analyze its composition, phase currents and voltages, then build a vector diagram. After that, depict the active along the horizontal axis, and the reactive - along the vertical and connect the ends of these vectors with the resulting vector - you will get a power triangle.

It expresses the ratio of active and reactive power, and the vector connecting the ends of the two previous vectors will express the full power. All this sounds too dry and confusing, so look at the image below:

The letter P - indicates active power, Q - reactive, S - full.

The full power formula is:

The most attentive readers probably noticed the similarity of the formula to the Pythagorean theorem.

Units:

• P - W, kW (watts);
• Q - VAR, kVAr (reactive volt-amperes);
• S - VA (Volt-amperes);

Calculations

To calculate the total power, use the formula in complex form. For example, for a generator, the calculation has the form:

And for the consumer:

But we apply knowledge in practice and we will figure out how to calculate the power consumption. As we know, ordinary consumers pay only for the consumption of the active component of electricity:

P = S * cos Φ

Here we see a new cos Ф value. This is a power factor, where Ф is the angle between the active and full components of the triangle. Then:

cos Φ = P / S

In turn, reactive power is calculated by the formula:

Q = U * I * sinF

To consolidate the information, check out the video lecture:

All of the above is true for a three-phase circuit, only the formulas will differ.

Answers to popular questions

Full, active and reactive power is an important topic in electricity for any electrician. As a conclusion, we made a selection of 4 frequently asked questions on this subject.

• What work does reactive power do?

Answer: it does not perform useful work, but the load on the line is full power, including taking into account the reactive component. Therefore, in order to reduce the overall load, they are struggling with it or, speaking in a competent language, compensate.

• How is it compensated?

- For this purpose, use a reagent compensation device. It can be capacitor units or synchronous compensators (synchronous motors). We considered this issue in more detail in the article:https://our.electricianexp.com/en/kompensaciya-reaktivnoj-moshhnosti.html

• Which consumers are causing the reagent?

- This is primarily electric motors - the most numerous type of electrical equipment in enterprises.

• What harms a large consumption of reactive energy?

- In addition to the load on the power lines, it should be borne in mind that enterprises pay full power, and individuals pay only active power. This leads to an increased amount of payment for electricity.

The video provides a simple explanation of the concepts of reactive, active and full power:

This is where we conclude our consideration of this issue. We hope that now it has become clear to you what active, reactive and apparent power are, what are the differences between them and how each value is determined.

Related materials:

• What is a power limiter for?
• Phase and line voltage in three-phase circuits
• How to determine the power consumption of electrical appliances